1. Field
The present disclosure relates to an automatic transmission and a method for controlling the automatic transmission.
2. Description of the Related Art
An automatic transmission capable of providing 8-forward-speed and 1-reverse-speed transmission using a four planetary gear sets and six engagement mechanisms formed from clutches and brakes has been developed (refer to, for example, Japanese Unexamined Patent Application Publication No. 2012-97864).
The automatic transmission described in Japanese Unexamined Patent Application Publication No. 2012-97864 includes an input shaft rotatably supported by a casing and an output member formed from an output gear disposed so as to be concentric with the input shaft inside the casing. Rotation of the output member is transferred to the right and left drive wheels of a vehicle via a differential gear or a propeller shaft.
First to fourth planetary gear sets are disposed in the casing so as to be concentric with one another. The first planetary gear set is a single-pinion planetary gear set including the following three elements: a first sun gear, a first ring gear, and a first carrier that supports a first pinion meshed with the first sun gear and the first ring gear so that the first pinion is rotatable about its axis and is revolvable around the first sun gear. When the carrier is fixed and if the sun gear is rotated, the ring gear rotates in opposite direction to the sun gear rotation. Accordingly, a single-pinion planetary gear set is also referred to as a “minus planetary gear set” or a “negative planetary gear set”. Note that when the ring gear is fixed and if the sun gear is rotated, the carrier and the sun gear rotate in the same direction.
Referring to a collinear diagram (a diagram indicating relative rotational speed ratios among three elements, that is, a ring gear, a carrier, and the ring gear, in the form of straight lines (speed lines)) of the first planetary gear set, if first, second, and third elements are sequentially disposed from one side of the collinear diagram, the first element represents the first sun gear, the second element represents the first carrier, and the third element represents the first ring gear.
Like the first planetary gear set, the second planetary gear set is a single-pinion planetary gear set including the following three elements: a second sun gear, a second ring gear, and a second carrier that supports a second pinion meshed with the second sun gear and the second ring gear so that the second pinion is rotatable about its axis and is revolvable around the second sun gear. If the three elements of the second planetary gear set are referred to as a fourth element, a fifth element, and a sixth element sequentially disposed from one side of the collinear diagram of the second planetary gear set, the fourth element represents the second ring gear, the fifth element represents the second carrier, and the sixth element represents the second sun gear.
Like the first planetary gear set, the third planetary gear set is a single-pinion planetary gear set including the following three elements: a third sun gear, a third ring gear, and a third carrier that supports a third pinion meshed with the third sun gear and the third ring gear so that the third pinion is rotatable about its axis and is revolvable around the third sun gear. If the three elements of the third planetary gear set are referred to as a seventh element, an eighth element, and a ninth element sequentially disposed from one side of the collinear diagram of the third planetary gear set, the seventh element represents the third sun gear, the eighth element represents the third carrier, and the ninth element represents the third ring gear.
Like the first planetary gear set, the fourth planetary gear set is a single-pinion planetary gear set including the following three elements: a fourth sun gear, a fourth ring gear, and a fourth carrier that supports a fourth pinion meshed with the fourth sun gear and the fourth ring gear so that the fourth pinion is rotatable about its axis and is revolvable around the fourth sun gear. If the three elements of the fourth planetary gear set are referred to as a tenth element, an eleventh element, and a twelfth element sequentially disposed from one side of the collinear diagram of the fourth planetary gear set, the tenth element represents the fourth ring gear, the eleventh element represents the fourth carrier, and the twelfth element represents the fourth sun gear.
The first sun gear (the first element) of the first planetary gear set is connected to the input shaft. In addition, the fourth ring gear (the tenth element) of the fourth planetary gear set is connected to the output member.
In addition, the first carrier (the second element) of the first planetary gear set is connected to the second carrier (the fifth element) of the second planetary gear set and the third ring gear (the ninth element) of the third planetary gear set and, thus, a first connected member (the second element, the fifth element, the ninth element) is formed. Furthermore, the first ring gear (the third element) of the first planetary gear set is connected to the fourth sun gear (the twelfth element) of the fourth planetary gear set and, thus, a second connected member (the third element, the twelfth element) is formed. Still furthermore, the third carrier (the eighth element) of the third planetary gear set is connected to the fourth carrier (the eleventh element) of the fourth planetary gear set and, thus, a third connected member (the eighth element, the eleventh element) is formed.
In addition, the automatic transmission described in Japanese Unexamined Patent Application Publication No. 2012-97864 includes six engagement mechanisms formed from three clutches, that is, first to third clutches, and three brakes, that is, first to third brakes.
The first clutch is a wet multiple disc clutch. The first clutch is configured so as to be switchable between a connection mode in which the first sun gear (the first element) of the first planetary gear set is connected to the third connected member (the eighth element, the eleventh element) and a release mode in which the first sun gear is disconnected from the third connected member. The second clutch is also a wet multiple disc clutch. The second clutch is configured so as to be switchable between a connection mode in which the first sun gear (the first element) of the first planetary gear set is connected to the second ring gear (the fourth element) of the second planetary gear set and a release mode in which the first sun gear is disconnected from the second ring gear.
The third clutch is a wet multiple disc clutch. The third clutch is configured so as to be switchable between a connection mode in which the second sun gear (the sixth element) of the second planetary gear set is connected to the second connected member (the third element, the twelfth element) and a release mode in which the second sun gear is disconnected from the second connected member. The first brake is a wet multiple disc brake. The first brake is configured so as to be switchable between a fixed mode in which the third connected member (the eighth element, the eleventh element) is fixed to the casing and a release mode in which the third connected member is unfixed from the casing.
A second brake is a wet multiple disc brake. The second brake is configured so as to be switchable between a fixed mode in which the third sun gear (the seventh element) of the third planetary gear set is fixed to the casing and a release mode in which the third sun gear is unfixed from the casing. A third brake is a wet multiple disc brake. The third brake is configured so as to be switchable between a fixed mode in which the second sun gear (the sixth element) of the second planetary gear set is fixed to the casing and a release mode in which the second sun gear is unfixed from the casing.
According to the automatic transmission described in Japanese Unexamined Patent Application Publication No. 2012-97864, by setting each of the first brake, the second brake, and the third brake in a fixed mode, a first forward speed ratio is established. By setting the second brake and the third brake in the fixed mode and setting the third clutch in the connection mode, a second forward speed ratio is established. By setting the second brake and the third brake in the fixed mode and setting the second clutch in the connection mode, a third forward speed ratio is established. By setting the second brake in the fixed mode and setting the second clutch and the third clutch in the connection mode, a fourth forward speed ratio is established.
By setting the second brake in the fixed mode and setting the first clutch and the second clutch in a connection mode, a fifth forward speed ratio is established. By setting the first to third clutches in the connection mode, a sixth forward speed ratio is established. By setting the third brake in the fixed mode and setting the first clutch and the second clutch in the connection mode, a seventh forward speed ratio is established. By setting the third brake in a fixed mode and setting the first clutch and the third clutch in the connection mode, an eighth forward speed ratio is established. By setting the first brake and the third brake in the fixed mode and setting the second clutch in the connection mode, a reverse speed ratio is established.